Many liquid crystal display (LCD) panels filter light from a light source called a backlight to produce images on their display screen. Backlights illuminate the LCD from a side or from the back, and each pixel of the LCD filters the light differently to produce a picture. Backlights can be provided in various colors. For example, color LCD displays may use white backlights, and monochrome LCD displays can have red, yellow, green, blue or white backlights. The backlight can usually be adjusted to produce a light level in a range from dark to full brightness. The level of full brightness depends on the backlight.
A light emitting diode (LED) backlight source can also improve the color range of a LCD display. For example, a LED white light can produce a color spectrum closely matching the color range of the LCD pixels so each color pixel can allow only the desired light spectrum through. This improves the light transmission efficiency of the display since only selectively desired light is produced, and brighter colors can be provided.
Frame rate refers to the frequency at which an imaging device produces unique consecutive images (frames). Frame rate is most often expressed in frames per second or Hertz (Hz). The higher the number of frames per second, the smoother the video appears to the user. Lower frame rates typically result in lower video quality and higher rates typically yield better video quality. As a reference, motion pictures typically use 24 frames/second (24 Hz), the American TV standard (NTSC) uses 60 frames/second (60 Hz), and the European TV standard (PAL) uses 50 frames/second (50 Hz) to allow the viewer to perceive smooth playback.
The refresh rate or vertical refresh rate for a LCD screen refers to the number of times per second (Hz) that the display hardware redraws the image on the screen. This is distinct from the frame rate because a relatively faster refresh rate can allow redrawing of identical frames, while frame rate measures the rate that a video source sends a new frame. For example, movies may have a frame rate of 24 frames per second, but each frame may be drawn (i.e., refreshed) two or three times on a LCD screen before the next frame is presented. Therefore, a movie running at 24 frames per second can have a 48 or 72 Hz refresh rate. Both the refresh rate and frame rate are controlled by LCD timing signals referred to herein as a refresh signal and a frame signal, respectively.
LCD screens may experience a number of problems which are at least partially due to backlighting, such as flickering, shimmering and banding. For example, flickering can be caused when a LED drive signal frequency is relatively slow compared to the frame rate of a LCD screen. In such situations, there may be substantial portions of a frame that are not backlit at a given instant in time. FIG. 1A illustrates one period of an exemplary LED drive signal 102 and two periods of an exemplary LCD refresh signal 104 (also known as a vertical synchronization signal 104). Note, in this example, two periods of the LCD refresh signal 104 corresponds to one frame of an image or picture. As shown in FIG. 1A, the second half of the image frame will have no backlight and, hence, will appear darker than the first half of the image frame. This leads to a blinking or “flickering” effect that is undesirable.
As shown in FIG. 1B, when the LCD refresh signal 104 is out of phase with the LED drive signal 102, additional undesired visual effects may appear in the display, such as shimmering and banding. As its name implies, shimmering resembles a “sparkling” effect that typically occurs when a moving dark object meets or intersects with a lighter one. For example, when tree leaves are blowing in the wind, the leaves may appear to artificially shimmer at the edges of the leaves. The cause of shimmering is similar to that of flickering but is further caused by a phase offset 106 between the LED drive signal 102 and the LCD refresh signal 104, as shown in FIG. 1B. Shimmering typically occurs when this phase offset 106 drifts or changes in time. Banding is similar to shimmering but the phase offset 106 does not drift or change in time, which creates stationary bands of dark areas that are typically evenly spaced across the display screen.
Although LCD display screens may be backlit by fluorescent lights or electroluminescent panels, light emitting diodes (LED's) are increasingly being used to provide backlighting because they are a more efficient and durable method of lighting. LED's have a long operating life, relatively low power consumption, and a broad color range. Therefore, there is a need to provide a method and LCD display that eliminates or reduces some of the problems associated with using LED backlights, such as flickering, shimmering and banding.